Alkylated aromatic compounds, such as cumene, ethylbenzene and sec-butyl benzene, are often produced by the liquid phase alkylation reaction of alkylatable aromatics (e.g., benzene) and alkylating agents (e.g., olefins such as ethylene, propylene and butylene) in the presence of acidic molecular sieve catalysts (e.g., zeolites). Liquid phase aromatics alkylation processes often result in reduced operating costs and fewer undesirable byproducts produced (e.g., xylenes) than in earlier vapor phase technologies.
Acidic molecular sieve catalysts that may be used for such liquid phase aromatic alkylation reactions include zeolite beta, zeolite Y, zeolite omega, ZSM-5, ZSM-12, MCM-22, MCM-36, MCM-49, MCM-56, MCM-58, MCM-68, UZM-8, faujasite, Mordenite, porous crystalline magnesium silicates and Tungstate-modified zirconia (e.g., Zr(WO4)2, all of which are known in the art.
Operation of liquid phase aromatics alkylation reactions especially at relatively low temperatures has resulted in greater catalyst sensitivity to trace impurities (e.g., “catalyst poisons”) in the alkylatable aromatic or alkylating agent feed streams. Such impurities often result in more frequent catalyst regeneration requirements and reduced ultimate life of the catalyst before replacement is necessary. Catalyst replacement often involves a process shutdown, lost production and significant costs. A variety of processes have been developed for pretreating aromatic and/or alkylating agent feed streams to remove catalyst poisons. These processes include distillation, adsorption and extraction.
U.S. Pat. No. 6,313,362 (Green) teaches an aromatic alkylation process in which the alkylation product is contacted with a large pore molecular sieve catalyst such as MCM-22 in a liquid phase step to remove impurities prior to liquid phase alkylation. Impurities taught as being removed include olefins, diolefins, styrene, oxygenated organic compounds, sulfur-containing compounds, nitrogen-containing compounds and oligomeric compounds.
U.S. Pat. No. 4,358,362 (Smith) teaches a method for enhancing catalytic activity of a zeolite catalyst by contacting a feed stream which contains a catalytically deleterious impurity with a zeolitic sorbent. This disclosure uses a sorbent with a Si/Al ratio greater than 12, 10-12-membered rings, and a Constraint Index between 1 and 12, preferably ZSM-11.
U.S. Pat. No. 5,030,786 (Shamshoum) teaches a process for production of ethylbenzene in which the catalyst lifetime is increased by reducing the concentration of water in the feed to the reactor.
U.S. Pat. No. 5,744,686 (Gajda) teaches a process for the removal of nitrogen compounds from an aromatic hydrocarbon stream by contacting the stream with a selective adsorbent having an average pore size less than about 5.5 Angstroms. The selective adsorbent is a non-acidic molecular sieve selected from the group consisting of pore closed zeolite 4A, zeolite 4A, zeolite 5A, silicalite, F-silicalite, ZSM-5 and mixtures thereof.
A process for preparing alkylated benzenes is taught in U.S. Pat. No. 6,297,417 (Samson). The process includes contacting a benzene feedstock with a solid acid, such as acidic clay or acidic zeolite, in a pretreatment zone at a temperature between about 130° C. and about 300° C. to improve the lifetime of the alkylation and transalkylation catalyst.
U.S. Pat. No. 6,355,851 (Wu) teaches a zeolite-catalyzed cumene synthesis process in which benzene feedstock is contacted with a “hot” clay bed, followed by distillation of the benzene feedstock to separate the benzene from the higher molecular weight materials formed from olefinic poisons during the hot clay treatment, followed by a “cold” clay treatment wherein the benzene distillate is contacted with an ambient-temperature clay. The propylene feedstock is pretreated by contact with an alumina to remove trace sodium compounds and moisture, a molecular sieve to remove water and two modified aluminas to remove other catalyst poisons. The pretreated propylene and benzene feedstocks are then reacted in the presence of a zeolite catalyst to form cumene without causing rapid degradation of the catalyst's activity.
PCT published application WO0214240 (Venkat) teaches removal of polar contaminants in an aromatic feedstock by contacting it with molecular sieves with pore size greater than 5.6 Angstroms at temperatures below 130° C.
U.S. Pat. No. 6,894,201 (Schmidt) teaches removing nitrogen compounds from an alkylation substrate such as benzene prior to alkylation using a conventional adsorbent bed which adsorbs basic organic nitrogen compounds and a hot adsorbent bed of acidic molecular sieve which adsorbs weakly basic nitrogen compounds such as nitrites. Schmidt teaches that water facilitates the adsorption of the weakly basic nitrogen compounds and that running an alkylation substrate stream from a fractionation column of elevated temperature and suitable water concentration to the hot adsorbent bed may be advantageous.
U.S. Pat. No. 7,199,275 (Smith) teaches a process for hydrocarbon conversion in which a partially dehydrated hydrocarbon feedstock is contacted with at least two different molecular sieve materials, including a first molecular sieve having a Si/Al molar ratio of less than about 5 and a second molecular sieve having a Si/Al molar ratio of greater than about 5. Also, Smith teaches processes in which such feedstocks are contacted with a first molecular sieve having pores of at least about 6 Angstroms and a second molecular sieve having pores of less than about 6 Angstroms.
These prior references do not provide for the alkylation of a feed stream by contact with an alkylation catalyst, wherein the feed stream contains an alkylatable aromatic compound, an alkylating agent and trace amounts of water and impurities and a portion of the water and impurities are removed at the same time as the feed stream is alkylated. The presence of water and impurities in the feed stream negatively impact the catalytic activity and cycle length of the alkylation catalyst in alkylation processes.
Therefore, there is a need for an improved process for the production of alkylated aromatics by contacting such a feed stream with a first and then a different second alkylation catalyst to remove a portion of water and impurities and alkylate a portion of the alkylatable aromatic such that the adverse impact on the activity and cycle length on such alkylation catalyst by water and impurities is mitigated. This disclosure meets this and other needs.